The objective of the proposed research is to gain more knowledge of the significance and consequences of abnormal activities of purine metabolizing enzymes in lymphocytes of immunodeficient patients. The proposal consists of two related projects: I) Expanding the diagnostic value of measuring adenosine deaminase and ecto-5'-nucleotidase activities in lymphocytes of patients with immunodeficiency syndromes, and II) studying the regulation of purine nucleotide synthesis in human thymocytes and peripheral T cells in order to determine the role of purine metabolism in regulating the proliferation of human lymphocytes at different stages of differentiation. In the first project, the following experiments will be performed: a) Age-dependent profiles of lymphocyte adenosine deaminase and ecto-5'-nucleotidase activities will be determined, and the data obtained will be used in evaluating abnormal enzyme activities in immunodeficient patients, b) Ecto-5'-nucleotidase activity will be measured in human T cell subpopulation separated with a fluorescence-activated cell sorter employing monoclonal antibodies to helper and suppressor T cells. We will then determine whether the low ecto-5'-nucleotidase activity seen in T cells of hypogammaglobulinemic patients is due to abnormal proportions of normal T lymphocyte subpopulations, c) Epstein Barr virus transformed cell lines already established on obligate heterozygotes of X-linked agammaglobulinemia will be cloned in an attempt to prove the existence of two B lymphocyte populations (one with and one without both ecto-5'-nucleotidase activity and surface immunoglobulin). In the second project, we will determine: a) the capacity of thymocytes and peripheral T cells to synthesize purines de novo (when purine salvage is minimized by the use of purine-free medium) as measured by 14C-formate incorporation into newly synthesized purines, b) the capacity of the purine salvage pathway in thymocytes and peripheral T cells utilize exogenous nucleotides, nucleosides, and bases when purine synthesis de novo is blocked with aminopterin, and c) the balance between purine synthesis de novo and purine salvage in supplying the purines required for lymphocyte proliferation by measuring the concentrations of exogenous purines needed to inhibit de novo synthesis.